The purpose of this proposal is to develop a modification of Teller's forced-choice preferential (FPL) looking procedure, to allow clinical assessment of visual acuity in human infants. Four types of studies are proposed: 1. Clinically related studies to be conducted in the laboratory. Preliminary norms for monocular testing will be established on presumptively normal infants born at term. Infants born prior to term and infants refered by eye practitioners will be tested to see whether infants who are presumed to have delayed visual development or impaired vision fail to perform as well as normal infants. 2. Testing of infants in clinical settings. The clinical usefulness of the procedure and the applicability of preliminary laboratory norms to patient populations will be assessed through tests of infants seen at a local well-baby clinic and through tests of patient populations of eye practitioners in various parts of the country. 3. Basic laboratory studies. Parametric studies will be carried out to assess the influence of variations in stimulus conditions on infant acuity, and to define the extent to which the various parameters should be controlled during clinical testing. 4. Apparatus development. Modifications in the existing apparatus will be tested. The purpose of these modifications will be to reduce the personnel required to test an infant and to develop a hand-held apparatus for vision screening under conditions in which the more formal clinical FPL procedure would be difficult to implement.